To test uploading a file without using fixture_file_upload, a posted
ActionDispatch::Http::UploadedFile should not be paramified (just like
Rack::Test::UploadedFile).
(Rack::Test::UploadedFile and ActionDispatch::Http::UploadedFile don't
share the same API, tempfile is not accessible on
Rack::Test::UploadedFile as discussed in
brynary/rack-test#30)

Backport of #6441cb6f839 . Old 'wait_timeout' is still supported,
but conflicts with mysql2 using that spec key for different thing.
'checkout_timeout' can now be used taking precedence for ConnectionPool
over 'wait_timeout'.